The following is from Supermicro support
Dropping this here should it be helpful to anyone else.
2024-12-03_10-51-16
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FROM: support@supermicro.com
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SUBJ: RE: Support question regarding X12STL-F MoBo [BT]
Thank you for contacting Supermicro Support.
Unfortunately, X12STL-F does not support PCIe bifurcation.
None of the X12STH boards do either.
Some of the X11SS_ (X11SSH-(LN4)F, X11SSL-F, and X11SSM-F) server boards support bifurcation.
None of the X11SC_ server boards support bifurcation. None of the X12ST_ boards support bifurcation.
X13SCL-IF x16 slot can be bifurcated to x8/x8 via jumper. The other X13SC_ boards do not support bifurcation.
If searching motherboard user manual cannot find “x8x8” or “x4x4” string, then it is likely the motherboard does not support PCIe bifurcation.